A stent for placement in living body is a generally tubular medical device which, for treatment of various diseases caused by stenosis or occlusion of a blood vessel or other living body lumen, is indwelled in the stenosed or occluded portion of the living body lumen so that the portion is dilated to secure the lumen.
An application involving use of the stent in a blood vessel is now described as an example, though the use of the stent is not limited in this regard.
A stent is inserted from outside into the inside of a living body, for which the stent is so designed as to be made relatively small in diameter (contracted state) at the time of insertion and to be expanded at an intended stenosed or occluded portion to achieve an enlarged diameter and maintain the lumen in an open state.
The usual practice is to use, as a stent, metallic wires or pipes processed into a cylindrical form. A stent is mounted on a catheter or the like in a radially reduced state, inserted into a living body, and expanded at an intended portion or the desired site in the lumen of the living body by some sort of method, thereby allowing the stent to be fixed in position close contact with the inner walls of the lumen and thus the shape of the lumen to be maintained. Stents are classified into self-expandable stents and balloon-expandable stents depending on the function and the manner of indwelling. A balloon-expandable stent has no expandable function by itself. After insertion of the balloon-expandable stent mounted on the balloon into the intended portion, the balloon is dilated or inflated so that the stent expands (or plastically deformed) by the action of the dilation force of the balloon, thereby permitting the stent to be fixed in close contact with the inner surfaces of the intended lumen. This type of stent needs the stent-expanding operations as mentioned above.
On the other hand, a self-expandable stent is one imparted with a self-expanding function in itself. This self-expandable stent is accommodated in a sheath in a radially contracted state and inserted into a living body, under which as soon as the stent is released from the sheath at an intended portion or desired site, it returns to an original expanded state by itself and is fixed in close contact with the inner walls of the lumen, thereby maintaining the shape of the lumen.
The current purpose of indwelling a stent is to return a blood vessel stenosed or occluded for some reason(s) to its original state of patency, mainly to prevent or reduce restenosis, which might occur after such a procedure as of PTCA in most cases. In recent years, in order to suppress the probability of restenosis to a greater extent, a drug-eluting stent having a drug such as an immunosuppressor or anticancer drug loaded thereon is used as well, and its effect is generally known.
On the other hand, as to the treatment of acute coronary syndromes, typical of which are acute myocardial infarction and unstable angina, or the treatment of unstable plaque that has been accepted as a preclinical stage thereof, no method therefor has been established yet. With respect to the treatment of acute coronary syndromes, almost all of existing stents and drug-eluting stents remain contraindicated. This is for the reason that if a stent is indwelled in a blood vessel containing a large amount of thrombi, the risks of stent malapposition and long-term thrombosis are undeniable.
With regard to the unstable plaque, MELER et al (Heart 2004; 90: 1395-1398: Plaque Sealing by Coronary Angioplasty) have proposed the concept of plaque sealing, in which the plaque is stabilized by giving a stimulus to its surface such as by balloon dilation. Recently, it has been reported that plaque sealing is carried out by use of a self-expandable stent having a relatively weak expansion force instead of a balloon.
Most of the self-expandable stents are employed in peripheral regions such as blood vessels of inferior limbs and carotid arteries, and, in the coronary region, the Radius stent alone, made by Boston Scientific Corporation, was previously introduced into the market. This stent has a form such as shown in JP-T-H11-505441 and International Application Publication No. WO96/26689. In this type of stent, stent indwelling positioning is more difficult than in the case of balloon-expandable stents in view of the properties of the stent, and it has been reported that there occurs a so-called jumping phenomenon in which the stent is unintentionally released from the sheath and indwelled.
It has been reported in academic conferences that when conventional balloon-expandable stents and self-expandable stents having a strong expansion force have been used in plaque sealing, there is the danger of rupturing the plaque owing to the stent indwelling operations themselves; and once the plaque has been ruptured, there is concern that a risk of peripheral occlusion and an increased inflammatory reaction thereat may occur and thus, such stents are unsuited for the plaque sealing.
Another type of stent has been proposed as described in Japanese Application Publication No. 2003-93519, U.S. Pat. No. 6,818,013, U.S. Pat. No. 7,037,331, and U.S. Pat. No. 7,311,726.
The stent disclosed in this patent literature includes a plurality of wave-shaped struts extending in an axial direction from one end side to the other end side of a stent and arranged in a circumferential direction of the stent, and a plurality of connection struts that interconnect the respective adjacent wave-shaped struts to each other and extend over a predetermined length, wherein the end portions of the respective wave-shaped struts join to the end portions of the adjacent wave-shaped struts. Since this stent is made of plural wave-shaped struts extending in the axial direction of the stent, the stent is flexible and has the possibility of application to the plaque sealing.
The connection struts of the stent described in Japanese Application Publication No. 2003-93519, U.S. Pat. No. 6,818,013, U.S. Pat. No. 7,037,331, and U.S. Pat. No. 7,311,726 extend in an axial direction while curving. According to our studies, we have found that this construction of the stent, including the connection struts extending in the disclosed manner, results in deformability under compression in a radial direction that is not satisfactory and an expansion force that is also not satisfactory.